The present invention relates to steels for nitriding, steel products for nitriding, nitrided steel parts, and manufacturing methods of nitrided steel parts. In particular, the present invention relates to manufacturing methods of nitrided steel parts, having high tensile strength, high fatigue strength, and excellent bending toughness, such as nitrided crankshafts for automobiles, industrial machinery, and construction machinery, through nitriding without thermal refining, to thus-manufactured steel parts, and to steel products for nitriding, and steels for nitriding having a specific chemical composition, serving as steel stock for the manufacture of such steel parts.
In manufacture of steel parts for automobiles, industrial machinery, and construction machinery, billets of carbon steels and alloy steels for machine structural use are formed into desired shapes through hot working such as hot forging, followed by (a) thermal refining to obtain a desired strength (herein, "thermal refining" refers to "quenching and tempering," "normalizing," or "normalizing and tempering") and, as needed, (b) surface hardening to impart a desired surface hardness to the thermally refined steel parts. Surface hardening (b) is intended to improve the fatigue strength, seizure resistance, and galling resistance of those parts that have undergone thermal refining (a). Regardless of whether or not surface hardening (b) is performed after thermal refining (a), thermally refined steel parts may be machined so as to assume their final shapes. When surface hardening (b) is performed after thermal refining (a), surface-hardened steel parts may be polished or ground, so as to assume their final shapes.
Specific examples of surface hardening include carburizing and quenching, induction hardening, flame hardening, and nitriding (including soft-nitriding). In carburizing and quenching, induction hardening, or flame hardening, a steel part is quenched from a high-temperature zone of austenite to thereby be surface-hardened. The thus-quenched steel part suffers the occurrence of quenching distortion and may suffer the formation of a quenching crack.
Thus, for surface hardening of a steel part, whose distortion must be particularly small, nitriding is employed.
As an example of steel for nitriding, SACM645 (aluminum-chromium-molybdenum steel), which specified in JIS G 4202, is well known. However, due to addition of a large amount of Al and Cr, which improve the effect of nitriding, SACM645 involves a problem that melting, casting and hot working are relatively difficult to perform.
Steel parts for automobiles, industrial machinery, and construction machinery must have small distortion. To this end, these parts tend to undergo thermal refining and then nitriding. However, in recent years, so-called "eliminating thermal refining" has been studied in order to reduce cost through elimination of thermal refining which was formerly performed before nitriding. (Hereinafter, nitriding which is not preceded by thermal refining is referred to as "nitriding without thermal refining".)
However, when ordinary carbon steels and alloy steels for machine structural use, such as SCM435 and SACM645, as defined by JIS, are nitrided without first being subjected to thermal refining, a coarse microstructure that forms during hot working, such as hot forging, remains in the final products, i.e. machinery steel parts. Accordingly, steel parts that have undergone nitriding without thermal refining involve a reduction in fatigue strength and bending toughness.
Japanese Patent Application Laid-Open (kokai) No. 8-170146 discloses a technique for nitriding without thermal refining.
However, the lower limit of fatigue strength (fatigue limit), which the disclosed technique aims to achieve, is 38 kgf/mm.sup.2 (373 MPa). Accordingly, this technique is not satisfactory when steel parts must have a higher fatigue strength.